Napalm bomb fuzing system; Fuze AN M173A1 with AN M23A1 white phosporous igniter USA

[pzgr40]

Cutaway model of an AN M173A1 or M918 all-ways impact fuze with an AN M23A1 white phosphorous igniter.
This combination was used to ignite the M77 series Napalm bombs.
The white phosphorous housing is screwed into a flange in the nose and the tail cone of the sheet aluminium bomb housing (see pict 03).

The Fuze AN M173A1 consists of a machined zinc fuze housing (1 ), threaded on the outside on both the wide part and the reduced part. The wide threaded part is screwed into the AN M23A1 igniter housing, the reduced diameter thread receives a middle piece (2 ) that houses the all-ways fuze mechanism. To the base of this middle part the booster (3 ) is screwed, containing 12 grams of teryl
Both the base of the fuze housing and the lower part of the hole in the middle piece are machined conical.
In the centre of the fuze housing a hole is drilled that has fine thread. In it, a threaded brass pin (4 ) is screwed which is connected to a propeller (5 ). The brass pin protrudes into the firing pin housing (6 ) of the all-ways fuze mechanism, forcing two steel balls (7 )outward into a groove in the upper edge of the firing cap housing (8 )of the all-ways fuze mechanism. They are -apart from the steel balls- kept apart by a weak spring (9 ).
When carried under the planes wing , the propeller is kept in fixated position by a safety pin that prevents the fuze from arming before being released. Upon release the pin is withdrawn, the propeller rotates, unscrewing the brass pin from the firing pin housing. After approximately 20 revolutions this releases the two balls that fall inward and arm the all-ways fuze. Upon impact of the napalm canister with the ground, the firing pin will be driven into the firing cap (red), indifferent of the position in which the napalm bomb will hit the ground. If head on, the firing cap is driven into the firing cap, when sideways, both the firing pin and firing cap housing will climb the conical slopes, forced so by inertia, forcing the firing pin into the firing cap. The firing cap ignites the detonator (10 ), which detonates the Tetryl booster (11 ). This will ignite the 0,567 kg charge of white phosphorous (12 ), that ignites the Napalm from the ruptured container.

The AN M23A1 igniter is a sheet steel pressed housing (13)to which a pressed sheet steel fuze well (14) is brassed. In the base a threaded filler hole (15 ) is brassed, closed by a plug screw. The lower part of the housing is threaded so it can be screwed into the front and back flange of the BLU-27 Napalm fire bomb.

Maximum diameter : 100mm over the thread
Height AN M23A1 housing : 96mm
Total height with fuze : 114mm
Length of AN M173A1 fuze : 116 mm

When searching for the term napalm bomb Vietnam on you tube one can clearly see the ignition of the phosphorous ignited by the white smoke stripes shooting away from the impact point.

Regards, DJH.

 

[Yodamaster]

Hum, hum, sorry but on your two pictures, the fuze on the M23 igniter is not an FMU 7 serie but an AN-M157 or 173 all-ways fuze !



Yoda

 

[pzgr40]

Hi Yoda, you are absolutely right. Thank god Erik send me the official TM, as it is very difficult to find info on the internet if you do not have the correct nomenclature.
I will edit the text.
Thank you for your input!.

Regards, DJH

PS: I see the title still contains the wrong name : FMU-7/b instead of M173A1. Maybe the moderator would be as kind as to change this , as I cannot.

 

[Sprockets]

Hello, DJH,

I am a bit mystified about where the fuze and igniter are fitted, as the dwg of the napalm bomb shows tail and nose cones in situ, which I can't think were just for carriage, removed when arming. I know that some napalm bombs had cones covering the fuzes, which were ejected upon release of the weapon. However, I can't think that this system would work with this fuze. Tell me where I am missing the point-as I always seem to do when querying your work! You also have an e-mail, in case it entered "Spam".



Martin.

 

[weberoed]

The FMU 7 is the only fuze that fits the M23 igniter, if I recall correctly the M 173 fuze fits the M 16 igniter, which is a side fitting WP burster. I'll he to double check tM12and 16, but I am reasonably sure I am correct. And yes the M 23 igniter with fuze is under the nose and tail section of the napalm bomb and is not exposed. I have seen a lot of pictures that show the M 23 with the M 173, but my bomb charts say it is wrong

 

[HAZORD]

The smaller (750 Lb. Mk.77 Fireye) Nape canister has flat spots on both ends, which has a recessed area that is threaded to engage the external threads on the WP igniter. I'll post some photos in a few hours.

The U.S. used the smaller ones in the first Gulf War to ignite the trenches filled with oil that Sadam had constructed. I believe dropped from Harriers.

 

[Sprockets]

Napalm BLU-27 fuzing.

Hello, pzgr40,

I think that the fuze used on the BLU-27 Napalm Bomb-R.H.Picture (Tapered Ends), may be based on that shown in US patent 3618524-dated 1968. Your fuze would not arm because the vanes would be shielded from the air flow, and sometimes the shortest arming period obtainable with a propellor would not allow for 50 foot drops. This patent shows a thermal battery at the middle of the bomb's length being operated, after a delay, by a firing pin released on drop. The current is then fed to the end mounted all-ways fuzes, releasing the arming pins via electrically triggered expansion bellows and release detents. You can see a central mounting for the thermal battery unit, triggered by a short arming wire on drop. Tell me, for just this once, that I am correct?! Also, I think that the phosphorous burster may be mounted externally so as to ensure that particles of the burning phosphorous are not smothered by the napalm preventing air access.
There is an urgent PM for you.

Martin.